The present invention relates to novel solventless reactive systems curable at room temperature, based on blocked polyisocyanates, organic amine with at least two primary amino groups, compounds with oxirane groups, and 2,3-dimethyl-3,4,5,6-tetrahydropyrimidine, and to processes for the preparation of these solventless reactive systems curable at room temperature.
The present invention further relates to the use of these solventless reactive systems for the manufacture of adhesives, sealing compounds, casting compounds, mouldings or coatings, e.g. coatings for ballast tanks, curable at room temperature.
Polyamine/epoxy resin systems are distinguished inter alia by excellent adhesion to metal, very good chemical resistance and outstanding anticorrosive properties. In the case of solvent-containing formulations and powder coating systems, crosslinked films of high flexibility can be obtained by using epoxy resins of high molecular weight and/or polyaminoamides, e.g. based on dimeric fatty acids, as hardeners. Because of the low molecular weight of epoxy resins and the resulting high network density, coatings based on solventless liquid resins and solventless amine-type hardeners are brittle. Nowadays, therefore, solventless formulations are plasticized using e.g. tar substitutes like coumarone resins. Particularly when larger amounts of hydrocarbon resins are used, such coatings tend towards long-term brittleness as a result of migration of the non-functional constituents.
A good and lasting elasticization of epoxy resins can be achieved by combination with polyurethanes. Thus, in DE-A 2 338 256, for example, high-molecular amine-terminated polyetherurethane-ureas were prepared by reacting prepolymers containing free isocyanate groups with amines in highly dilute solutions and then hardened with epoxy resins.
The use of the solvents required for this purpose, especially aromatic solvents, is disadvantageous in practice from both technical and physiological points of view. On the other hand the viscosity of solventless reaction products, such as those specifically prepared according to DE-A 2 338 256, is too high for practical use.
DE-A 2 418 041 describes a process for the manufacture of elasticized mouldings and flat structures wherein epoxy compounds are reacted with amine compounds obtained by the hydrolysis of prepolymeric ketimines or enamines. Chemically resistant thermosets which adhere well and have improved properties can be prepared by this process.
However, the process described in said publication is elaborate in terms of process technology and hence expensive.
DE-A 2 152 606 describes reactive systems based on alkylphenol-blocked polyisocyanates and polyamines which can optionally also be hardened in combination with epoxy resins. These reactive systems again suffer from disadvantages in terms of application technology: On the one hand the reactive systems have a relatively high viscosity; on the other hand the blocking agent liberated is of comparatively low molecular weight and therefore migrates out of the coating over time, which can lead to adhesion problems. Moreover, the level of mechanical properties does not satisfy all the technical demands.
To facilitate a specific reaction of polyisocyanate prepolymers with excess amounts of diamine, it was therefore frequently proposed to use the polyisocyanates in blocked form, e.g. as described in CA-A 1 219 986, EP-A 293 110 or EP-A 82 983. A common feature of all these publications is that phenols or substituted phenols are used as the preferred blocking agents. After reaction with the polyamines, removal of these substances from the reaction mixture by distillation is impossible, or only partial, because of their high boiling point, but residues of the optionally substituted phenols in the mixture or in the plastic composition lead to the disadvantages already mentioned.
The last-mentioned publications do in fact indicate that, in principle, it is also possible to use the other blocking agents conventionally employed in polyurethane chemistry. Such blocking agents conventionally used in polyurethane chemistry are oximes, caprolactam, ethyl malonate and ethyl acetoacetate. As none of these blocking agents can be incorporated into the polymer skeleton during epoxy hardening, and such compounds are not normally used in conventional amine/epoxy chemistry, the use of such blocking agents in place of the preferred, optionally substituted phenols offers no great advantages.
By contrast, EP-A 457 089 uses secondary amines, preferably of low boiling point, as blocking agents. If these amines remain in the reaction mixture after deblocking, they easily create an odour nuisance. Although, after being introduced into epoxy systems, the secondary amine can in principle be incorporated into the system, this will happen relatively slowly, especially in the case of applications at low temperatures (e.g. room temperature), and part of the amines will escape.
In one particularly preferred application, the amine-type blocking agent is distilled out of the reaction mixture after deblocking. Although this procedure yields products which do not create an odour nuisance, it is very elaborate and hence expensive.
The object of the invention was therefore to provide reactive systems curable at room temperature, based on blocked polyisocyanates, polyamines and compounds with oxirane groups, which do not suffer from the disadvantages of the systems according to the state of the art.